Despite considerable progress in diagnosing and treating solid tumors, metastatic disease remains the foremost cause of cancer-related death. Although the mechanisms of metastasis development are yet to be fully elucidated, the circulation of tumor cells derived from the primary tumor in the bloodstream of a patient is a fundamental intermediate event in the metastatic cascade. Circulating tumor cells (CTCs) in peripheral blood of cancer patients can be reliable biomarkers for detection of, e.g., metastasis and earlier detection of secondary tumors as well as monitoring of, e.g., disease progression and/or response to therapy. Despite their high phenotype heterogeneity, it remains challenging to capture CTCs with high efficiency and high purity because such a relatively small number of CTCs are present in the blood (e.g., about 1 CTC per 109 cells in peripheral blood of patients with metastatic cancer).
Size-based filtration allows for isolation of both epithelial and mesenchymal phenotypes, which are more appropriate for analyses of tumor heterogeneity, tumor drug resistance, etc. The main problem of this approach is contamination: the track-etched cylindrical holes of the filter have generally high retention ratio of non-tumor cells if the size of the holes is sufficiently small to reach high tumor cell capture efficiency.